1. Field of the Invention
The present invention relates to a semiconductor device using a semiconductor thin film and its manufacturing method and, in particular, to a thin film transistor (hereinafter referred to as TFT) using a crystalline semiconductor thin film containing silicon.
2. Description of the Related Art
In recent years, a technology for forming a TFT on a glass substrate or the like to constitute a semiconductor circuit has been rapidly advanced. As a typical semiconductor device, there is an active matrix type liquid crystal display (hereinafter referred to as AMLCD) integrated with a driver. The AMLCD integrated with a driver is a monolithic display device having a pixel section and a driver circuit on the same substrate. Further, a system-on panel in which a memory circuit, a clock generating circuit, and the like are built has been developed.
While a TFT in which an amorphous silicon (hereinafter referred to as a-Si) is used in an active layer is employed as a switching device of a pixel in a conventional AMLCD, in the peripheral circuit of the AMLCD integrated with a driver is mainly employed a TFT in which a polycrystalline silicon (hereinafter referred to as poly-Si) having a higher field effect mobility is used as the active layer because the a-Si is not suitable for the peripheral circuit which is required to operate at high speeds.
A conventional poly-Si TFT has a higher field effect mobility than an a-Si TFT. When a variety of circuits are mounted on a system-on panel, however, because the circuits are required to operate at higher speeds, the TFTs employed by the circuits are required to have a field effect mobility higher than the AMLCD integrated with a driver.
Also, even in the AMLCD integrated with a driver, TFTs having a higher field effect mobility are required because it is required to operate at high speeds due to an increase in the number of pixels and to reduce the area of a driver circuit.
Chief among factors determining the field effect mobility of the TFT is a surface dispersion effect. The flatness of an interface between the active layer of the TFT and a gate insulating film produces a large effect on the field effect mobility of the TFT, and as the interface becomes flatter, the effect of the surface dispersion becomes less and thus the field effect mobility becomes larger.
One of currently available methods of producing a crystalline silicon film is a laser crystallization method, and a method of applying an excimer laser to an amorphous silicon film to crystallize it has been known. An amorphous silicon film having a thickness of 10 nm to 150 nm (typically 30 nm to 60 nm) is formed on an insulating substrate by sputtering, CVD, or the like and subsequently irradiated with excimer laser light, whereby the amorphous silicon film is melted, solidified, and crystallized. In the case where the amorphous silicon film contains about 5% or more hydrogen, it is previously dehydrogenated by a heat treatment performed at temperatures of about 400xc2x0 C. to 500xc2x0 C. for several hours because hydrogen is explosively removed when it is annealed by a laser.
While the conditions of the laser crystallization are selected by an operator, when the excimer laser is employed, for example, a laser pulse oscillation frequency is 30 Hz and a laser energy density is 100 mJ/cm3 to 500 mJ/cm3 (typically, 300 mJ/cm3 to 400 mJ/cm3). A linear laser beam is applied to the whole surface of the substrate, wherein the overlapping ratio of the linear beam is 80% to 98%.
Protrusions (or bumps) called ridges are formed at random on the surface of the film crystallized in this way by the laser beam. It is thought that the protrusions are produced by surface tension waves induced on the surface of Si annealed and melted by the laser. Typically, the protrusions have a thickness two times the thickness of the thin film. The thickness of the thin film is usually 30 nm to 60 nm and thus the protrusions have a height of 30 nm to 60 nm from the surface of the film. The protrusions formed in this way give dispersion to the movements of electrons or holes due to the above-mentioned surface dispersion effect to reduce the field effect mobility of the TFT. The larger the protrusions are, the larger the effects are.
It is an object of the present invention to provide a technology for controlling the arrangement of the protrusions described above to reduce the effect of a surface dispersion on an electric current.
The present invention produces a crystalline semiconductor thin film by melting and solidifying a non-single crystalline semiconductor film such as amorphous, microcrystalline or polycrystalline semiconductor thin film by the use of strong light such as a laser or the like, and is characterized in that protrusions existing on the surface of the crystalline semiconductor thin film are aligned in parallel to the direction of length of a channel, that is, the direction in which an electric current flows to thereby produce an electric current path which is not affected by the surface dispersion caused by the protrusions.
In FIGS. 1(A) and 1(B) and FIGS. 2(A) and 2(B) are shown the conceptional view of the present invention. FIGS. 1(A) and 1(B) are a schematic view of the surface of a semiconductor thin film produced by crystallizing an amorphous silicon film by a conventional laser crystallization technology, whereas FIGS. 2(A) and 2(B) are a schematic view of the surface of a semiconductor thin film produced by the present invention, the respective figures showing a schematic view of a channel portion of a TFT. In the conventional technology, since protrusions 1001 are formed at random on the surface of a semiconductor thin film 1003 of a substrate 1004, a plurality of protrusions interfere with an electric current path 1002 to reduce mobility by the effects of surface dispersion. In the present invention, since the protrusions 1005 are aligned in parallel to an electric current path 1006, current paths which do not cross the protrusions 1005 are predominant. Such current paths are not affected by the surface dispersion and hence produce high field effect mobility. In other words, the arrangement of the protrusion is such that regions having a larger number of protrusions and regions having no or less number of protrusions appear in turn in a direction orthogonal to the electric current path 1006. The effect of the present invention may be obtained even if this direction is slightly deviated from the ideal direction, for example, within a range of xc2x130xc2x0, preferably xc2x115xc2x0.
Further, also in the case where a crystalline thin film produced by a thermal crystallization technology or the like is melted at least partly and solidified by strong light such as a laser or the like for the purpose of improving its properties, protrusions are produced as is the case where an amorphous film is melted, solidified and crystallized by strong light such as a laser or the like. In this case, the case where the protrusions existing in the crystalline thin film melted, solidified and re-crystallized are aligned in parallel to the direction in which an electric current flows to produce an electric current path not affected by the surface dispersion effect of the protrusions is also included in the present invention. The essential object of the present invention resides in intentionally aligning protrusions, which are produced when a thin film containing silicon is melted and solidified, in an objective direction, and the present invention is not limited by the property or the sort of a starting film.
A mechanism of producing the protrusions produced when a thin film containing silicon is melted and solidified by strong light such as a laser or the like has not yet completely clarified until now. It is considered true as described above, however, that the protrusions are caused by surface waves produced when the thin film is melted. The present inventor has tried controlling the positions of the protrusions by positively controlling the surface waves produced when the thin film is melted and solidified and thus has achieved the present invention. An idea of positively controlling the surface waves produced when the thin film is melted has never been tried and is the one of the features of the present invention.
When a uniform silicon thin film is melted, there is no cause for limiting the shapes of surface waves and hence the surface waves are produced at random, that is, protrusions are produced at random when the thin film is solidified, whereas when the thin film is melted such that the surface waves are aligned in a specific direction, the protrusions are also aligned along the direction in which the surface waves are formed. The present inventor has discovered that the wave front of the surface waves can be aligned in a specific direction by providing a structure of controlling the surface waves produced when the thin film is subjected to laser annealing.
To be more specific, the present inventor has realized the alignment of the wave front of the surface waves by forming a material, which has a thermal conductivity larger than a substrate including an underlying film, as a heat absorbing layer in a predetermined shape before a semiconductor thin film was formed. FIG. 3 is a schematic cross-sectional view of the heat absorbing layer. When a semiconductor thin film is subjected to laser annealing, a temperature difference is produced between a semiconductor thin film 1010 positioned above a heat absorbing layer 1011 formed on a substrate 1014 via an underlying film 1012 and a semiconductor thin film 1013 of the other region to produce a difference in thermal expansion at the boundary of the outside end 1015 of the heat absorbing layer. A difference in volumetric expansion caused by heat produces a strain starting at the boundary. This strain becomes a surface wave and propagates to form a surface wave starting at the outer periphery of the heat absorbing layer in the vicinity of the heat absorbing layer. When the semiconductor layer is solidified after it is melted, the protrusions of the surface wave remain as protrusions after the semiconductor film is solidified.
FIG. 4 is a SEM photograph of the surface of a poly-Si semiconductor thin film formed by using the present invention, and FIG. 5 is a schematic view of the surface of the semiconductor thin film. It can be observed that the protrusions 1022 are aligned like ripples on the semiconductor film 1021 around the semiconductor film 1020 formed on the heat absorbing layer at the center. FIG. 6 shows an AFM observation image in which the protrusions are aligned. FIG. 7 is a schematic view of the AFM observation image, in which a surface state 1051 of a region of 2.5 xcexcm square is shown. As is evident from the figure, protrusions 1054 are aligned and a cross-sectional shape 1058 along a direction 1056 parallel to the line of the protrusions has a surface having a smaller number of protrusions and dips than a cross-sectional shape 1057 along a direction 1055 perpendicular to the line of the protrusions. Here, the height of the cross-sectional shape is shown in a scale of about 90 nm in full range. In the surface state like this, flowing an electric current in the direction parallel to the line of the protrusions can produce an electric current path which is not affected by the surface dispersion and thus can realize a TFT having high mobility.
In a sample in accordance with the present invention, a 0.7 mm thick glass plate (No. 1737 made by Corning Corp.) was used as a substrate and a heat absorbing layer (300 nm) was formed of Ta (tantalum). A 125 nm thick silicon oxide film was formed as an underlying film by a PCVD and thereafter a-Si film having a thickness of 30 nm was formed by the PCVD. Then, the sample was dehydrogenated at 500xc2x0 C. for 1 hour and is subjected to laser annealing at a room temperature by applying 10 shots of a XeCl excimer laser at a power of 308 mJ/cm2.